The present invention relates to a pressure transducer assembly for a process circuit used in a process plant.
In process plants, such as chemical and refining plants, pressure transducers installed on the site are used as field instruments to measure fluid pressure. Pressure transducers are used for example to measure the fluid pressure and to detect the fullness of the fluid. Measuring diaphragms with differential pressure transducers are frequently used to ascertain fluid flow-through.
A generally known pressure transducer assembly for measuring fluid pressure includes at least one fluid measuring line going between the process circuit and the valve of the pressure transducer. It is furthermore known that for simple assembly and possibly disassembly, at least one of the measuring lines is connected by means of flange connections. For this purpose a flange connection between the process circuit and the fluid line is provided. The flange connection consists of a flange on the process circuit side and another appertaining flange on the fluid line side, which are then held together by nuts and bolts. A second flange connection between the valve and the fluid line is also provided. These flange connections consist of a flange on the valve side and a mating flange on the fluid line side. The flange connection is held together by nuts and bolts. The flange on the valve side may also be in the form of a contact plate.
In a known pressure transducer assembly, the pressure transducer is isolated from the process circuit because of high temperatures encountered in the process circuits. This isolation is accomplished by locating the transducer at a sufficient distance from the process circuit by using long fluid measuring lines which carry the fluid. Pressure transducers based on this model are expensive, since the pressure transducer and the valve must be attached separately at a suitable location and the long fluid lines must then be connected. When the process circuit carries a hot fluid, there is usually no overheating problem on the pressure transducer, since cooling takes place over the long measuring lines and an acceptable temperature is not exceeded in the pressure transducer. However, when long measuring lines are used there are risks that the fluid pressure results may be inaccurate thereby defeating the purpose of the transducer assembly. When the fluid lines for measuring the fluid pressure are shortened in order to increase the pressure measurement accuracy, the fluid may not flow continuously thereby causing the pressure transducer to overheat.
To reduce the cost of the pressure transducer assembly it is known to set up the pressure transducer assembly directly on the process circuit, whereby at least one short and stable measuring line can support the valve and the pressure transducer, and no other support is needed.
In such a cost cutting pressure transducer that is used on a process circuit that is carrying a hot fluid, problems can arise. The pressure transducer may overheat as acceptable maximum temperatures for the transducer may be exceeded because of the use of one or more short measuring lines. The hot fluids in the short lines are not cooled, but instead can transfer the heat to the transducer. Due to this overheating problem, process plants must rely on the more expensive known art of using long measuring lines and the separate attachments of the valve and pressure transducer.
Furthermore, a direct pressure transducer assembly on a hot process circuit has already been proposed (WO 97/22855, FIG. 2) in which the pressure transducer with relatively long measuring lines is installed on the process circuit and is connected to the latter through additional holding means. In spite of the direct installation of this pressure transducer assembly on the process circuit, the installation is still expensive, due to the necessary additional holding means, as the attachment only through the long fluid lines is not sufficiently stable.
It is an object of the present invention to provide a pressure transducer assembly of this type to be used on a process circuit in a process plant that carries a hot fluid in such a manner that the assembly is still simple and no overheating of the pressure transducer occurs. It is a further object of the invention to propose a pre-assembled apparatus suitable to this purpose.
The object of the invention is attained, according to the invention, by providing a thick, heat-insulating intermediate layer installed between a flange on the process circuit side and a flange on the measuring line side and/or between the flange on a valve side and a flange on the measuring line. Two or only one flange connection may be provided in this case, depending on the situation. Since at least one of the measuring lines, is a stub line and as such does not carry the hot fluid continuously, a heat transfer essentially occurs in the fluid""s stationary state while in the line between the process circuit and the temperature-sensitive electronic system of the pressure transducer. Thanks to the heat-insulating intermediate layer according to the invention on one or several of the flange connections, the resistance to a heat transfer is increased considerably at those locations so that with proper sizing the acceptable maximum temperature is not reached at the pressure transducer, even when short measuring lines are used for cantilevered support.
The installation of a heat-insulating intermediate layer between associated flanges of the flange connection is simple. This intermediate layer also has no detrimental influence on the supporting function of the short measuring line. The stub line can still support the valve and the pressure transducer, so that no additional holding means are necessary for their attachment.
A plastic gasket with a passage opening for the fluid is especially well suited as an intermediate layer, whereby the plastic is wedged sealingly or possibly with an additional seal ring between the associated flanges. A stop valve may be furthermore provided advantageously in the usual manner in the measuring line. The heating of a pressure transducer at a process plant is known and has been intentionally done on occasion to achieve certain effects. The cooling of a pressure transducer to unacceptably low temperatures during the cold season can cause various problems for a plant. Cold weather can bring the possibility of the fluid freezing in the measuring line or can cause the fluid to become viscous and thus yield the wrong measuring results. When a pressure transducer is heated these cold weather effects can thereby be avoided. To achieve this result a heat transfer that starts at the heater and goes along at least one of the measuring lines in the direction of the process circuit is necessary. For a pressure transducer where such heating is necessary due to individual conditions, the invention proposes not to insert heat-insulating intermediate layer in the flange connection close to the heat transmission. The heat-insulating intermediate layer would instead only be placed between the flange on the process circuit side and the associated first flange on the fluid measuring line. With suitable sizing and placement of the heat-insulating intermediate layers several problems can be solved. Overheating of the transducer when the process circuit is carrying hot fluid can thereby be avoided on the one hand, and on the other hand unacceptable cooling of the pressure transducer and of the measuring line can be countered when the process circuit carries cold fluids and/or in case of cold environmental temperatures. In addition, and to increase the resistance to possible overheating, bushings, and in particular flange bushings made of a heat insulating material may be provided in the pass-through openings of the flange through which the flange connecting bolts are inserted, so that the heat transfer via the flange bolts is also reduced. A pressure transducer assembly with a measuring diaphragm for the measurement of fluid flow-through may be used. The above transducer could have two fluid measuring lines, in which the heat-insulating intermediate layers according to the invention can also be used to good effect.
Placing a pressure transducer assembly at least partially within a two-part protective cabinet that can be opened is known. The cabinets have an internal holding device that protrudes into the inside and is attached to at least one cabinet part. It is preferably attached to a hinge element of a hinged model. This internal holding device is connected to at least one of the measuring lines and/or to the valve and/or to the flanges (DE 44 42 944 A1). According to the invention, the internal holding device is designed at least in part as a cooling body with large heat radiation surfaces as a further development of this pressure transducer assembly. Heat release by radiation for the protection of the pressure transducer is thereby achieved advantageously in addition to the increase in heat transfer resistance according to the invention.
A preassembled pressure transducer assembly incorporating the essential characteristics of the pressure transducer assembly described above may also be provided. This preassembled pressure transducer assembly with the heat-insulating intermediate layer or layers can be used on a process circuit in a process plant capable of carrying hot fluids.